Contents

Compensate for skew pulling

The tighter the wires, i.e. the more horizontal all wires are, the less tilt. (tested: true)

The tilt is smaller the closer to the centre, so spreading the support towers compensates. (tested:true)

Have the "heavy" parts hang below the wires, so it lessens the tilt. Will it swing like a pendelum? Can they be dampened? (first test inclusive of benefit)

The tilt is smaller(?)/less noticable if the platform is very small. Make it zero size and suspend the actual platform below. Same pendulum problems (tested:probably false)

Make the platfom active with servos compensating (a gyroscope frame)

That will probably make the head too large, expensive, heavy.

Let it be skew - it is possible to predict in the software and have the position offset so the sand hits the rights spot.

More than one wire to drag it horizontal, but still pulled by one motor. (Partial test: very promising)

Either two horizontal wires (i.e. look similar to a Rostock printer))

or two vertical wires (first brief test:promising)

The small prototype will be used to try most of above solutions

Dispenser

6 (maybe 10) dispensers with the dispensing hole beneath. Plan B is a single dispenser, and some snap-change of the sand container/dispenser.

Either the sand flows with gravity feed, and just need to open/close a valve, or the sand is metered out with a screw feed.

The wiring from the dispenser would go vertically up to a support arm overhead (fishingrod) Or for the fancy version the support wires would carry V+, GND and Serial, respectivly, toa boar that decodes and drives valves or feed motors.

Either the dispensers are on the plate (making it heavy) or they are on a solid support structure high overhead with a plastic tube down to the head. The latter avoids the weight penalty of many colours, but makes the whole plotter more bulky (and ugly) with the extra support structures.

Gravity feed

Valve
The sand flows freely until a coil activated valve closes. Or a plate with only one hole which moves to the right dispenser

No prototype, yet.

Vibrator
The sand does not flow of its own out of the dispenser. But when shaken it does. Attaching a small mobilephone vibrator makes it happen. Small handheld prototype worked fine - need to mount it for more real test.

Screw feed

This requires a motor for each feed; small and geared like continous rotation servos.

Kulitorum made a quick prototype with a nylon block and a drill bit. (and a big electric drill to drive it) The pitch is too steep. Have tried a little with sand and a spring, a little like left picture .

A 3D (to be printed) container to the right. Source is here in OpenSCAD and STL.

Purchase sand

How much to transport (air) down there? Once colours are mixed up we can not reuse it. (Unless colour is soluble in water and we can re-colour)

Fine sand from the beach (windblown, so reasonably fine and even) washed, dried and sieved. Experimenting with colouring it.

Member Jeff had some proper decoration sand which he has donated. Looks nice.

Initialisation

The maths for the positioning is absolute. On power up the machine needs to reset. During power off the wires probably unwind slightly (or a lot) and loose the correct tension.

A XY-plotter can just go to end stops, but this is not possible here, as we cant just unwind two wires and wind one (to get the first end stop) without the head possibly - depending on the start position - dropping to the surface or the wires go slack and tangled on the winding drums or overtightning. Thus:

A manual process with buttons controlling each motor indvidually to position the head "dead centre"

A rather tedious process. But then, it should not be needed too often. Needs almost a whole keyboard - control it through the PC.

Some stress/tension sensor, so it is possible to carefully nudge the head round to end stop (one or all 3?)